No CHECK ENGINE light was lighted and no stored codes were found in
the ECU. The O2 sensor signal does not fluctuate, stays at 0.68 volts,
and resistance measured as 12 ohms.
It seems that there is a hard failure of the O2 sensor, and yet no
engine trouble codes were found stored in the ECU memory.
At what temperature must the exhaust gases reach prior to testing an
O2 sensor on this vehicle?
While at the dealership, the vehicle was not put on a
dynamotor. The above test results were reported by the technician.
Is O2 sensor testing normally done after running vehicle on a
dynamotor to get components/exhaust gases up to proper temperature ?
Sensor is now on order via dealership service shop.
Service Advisor recommended that emissions re-inspection be delayed
after replacing the O2 sensor a couple of days to allow for "break
in".
Is a break in period applicable/required for an O2 sensor?
I have read that a CAT needs a break in period.
Do you agree that the re-inspection should be delayed ?
Thanks.
~ Vince ~

Testing an 02 sensor is an extremely easy bit for anyone vaguely familiar
with EFI systems. This should (but sometimes doesn't) include most
technicians. Especially at a dealership. And especially one asked to
diagnose an emissions failure problem.
Anyway, the car should be up to operating temperature. Exhaust gas
temperature is not a real variable here. No dynomometer need be involved.
All you need is a decent DVOM (digital volt-ohm meter). Simply probe the
connector at the o2 sensor and read the voltage being produced. Rev the
engine to around 2500 to get a good whoosh of gas through the pipes. It
should occilate from .2v to .8v about 8 times every ten seconds. Thats it.
Although I know service advisors love dispensing technical guidance, no
'break in' period need be allowed with a new sensor. Once the new sensor is
installed in the exhaust stream, it'll either produce the proper voltage or
it won't.

Well I work on Toyotas sometimes at my part time job. Sometimes
on some old Toyotas you will find a little black plastic box, when you
open the cover you should see a diagram of the wire terminals in the
black box. If you have a digital volt ohm meter you can set the meter
to dc volts and measure Ox with respect to ground, usually E1 which is
Earth 1 (British Earth is American Ground). Sometimes the voltage will
fluctuate so fast you look at the little bar graph if your meter has
one. Sometimes the box has a bad connection so you will have to probe
the oxygen sensor connector directly.
One wire oxygen sensors always have that wire carrying the o2
sensor signal to the computer and signal ground must be through sensor
body. 2 wire sensors are usally o2 sensor signal and ground to
computer. 3 wire is probably 02 sensor signal and ground through
sensor body with 2 other wires heater supply volts (usually 12V) and
heater ground. 4wire is probably 02 signal, 02 signal ground, heater
supply volts, and heater ground. So you must probe the right wire with
your voltmeter.
The old 1 wire sensors without heaters needed a long time to heat
up (5 minutes and up) and sometimes they would cool down and not work
during engine idle even after they warmed up. But the heated oxygen
sensors probably work within a minute or so after starting and they
didn't cool down during idle. The oxygen sensor needs to hit around
600 degrees fahrenheit before they work. With the old unheated sensors
the computer would let a timer run out and look at the coolant
temperature sensor to determine whether to go into closed loop (listen
to and believe oxygen sensor signal).
Since the oxygen sensor signal is stuck at 0.68 which for a normal
signal is rich. What causes the sensor signal to fluctuate is the back
and forth dance of the computer and oxygen sensor. The sensor sends a
lean signal (under about 0.45) and the computer richens mixture till
the oxygen sensor sends a rich signal (over about 0.45) and the
computer leans mixture, and does it over and over again. Sometimes a
computer doesn't want to go into closed loop because of defective
inputs from other sensors like the coolant temperature sensor or mass
air flow sensor.
Most time the Toyota manuals say to run the engine in park at
about 2000 rpm for 5 minutes and the oxygen sensors should be hot
enough to work. You shouldn't need a dynamometer, but even if you did
just run on street for 5 minutes or so. The oxygen sensor shouldn't
need a break in value but the computer to store long term block values
to use the oxygen sensor most efficiently may need a few days break in.

I noticed yesterday that the vehicle's clock indicated approximately 2
hours and 15 minutes behind the actual local time. This indicates
that the vehicle's electrical supply system (battery neg I would
think) was disabled for that accumulated amount of time.
I then asked myself why the electrical supply system had been
disabled? The only reason that I can think of myself is that
resistance measurements to evaluate the O2 sensor circuit were taken.
Any other reason than that one ?

The service department probably disconnected the battery to clear the ECU's
memory and turn off the MIL. I doubt if it was disconnected for that long.
More likely, the clock reset at 12:00 when they reconnected the battery. If
the shop did not write down your radio station presets, they are probably
gone as well.

No, it indicates the battery was disconnected only. The time
discrepancy is because the mechanic didn't set the time, and they
automatically start at either 12:00 or 1:00. A little math working
backwards should correspond to the time they were working on it.

There are many procedures where they tell you to disconnect the
battery even though the odds of something bad happening are
vanishingly slim - Sorry, but no big conspiracy here.
(And if a mechanic is working on a customer's car with the battery
still connected when it shouldn't be, and something DOES go very
wrong, he's going to be in a big steaming pile of trouble.)
Whenever you are working near the starter motor or other components
with high current supplies or unshielded power terminals, or on
computer system leads where you could accidentally fry something very
expensive, the FSM calls for a battery disconnect.
--<< Bruce >>--

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